A vehicle air conditioner may use an internally controlled variable displacement compressor (INT-COMP), or an externally controlled variable displacement compressor (EXT-COMP) to compress refrigerant. The INT-COMP, also known as a pneumatically-controlled or passively-controlled variable displacement compressor, varies compressor displacement based on refrigerant pressures within the compressor. The lowest operating displacement of the INT-COMP is limited to insure proper startup conditions, and so employs a clutch to disengage the INT-COMP when air conditioning is not requested or the load on the air conditioner is low. The EXT-COMP, also known as an actively-controlled variable displacement compressor, varies compressor displacement in response to a signal from a controller that is based on one or more environmental sensors. An INT-COMP is generally less expensive than an EXT-COMP because the INT-COMP does not need a controller or environmental sensors. However, an EXT-COMP is considered to be more energy efficient because the displacement or stroke of the compressor can be optimized for the environmental conditions and air conditioning demands at hand.
When an EXT-COMP is operated at moderate to high displacements, a large portion of the total energy consumed provides cooling, and only a small portion of the total energy is lost to reciprocating losses within the compressor. However, when operating at lower displacement levels, a greater fraction of the total energy is consumed by reciprocating losses, so the relative efficiency of the compressor is reduced when compared to the same compressor operating at higher displacement levels.
What is needed is a way to increase the operating efficiency of an externally-controlled variable displacement compressor when the compressor is operating at lower displacement levels.